In many manufacturing processes individual sheet metal parts are fabricated with locating holes. The sheet metal is positioned using pins which extend through these holes while welding of the parts occurs. Locating pins hold the sheet metal parts in position relative to each other and to the overall assembly. Accurate positioning of the pins assures that a consistent assembly is created during the welding process.
Some locating pins can be fixed to the frame of the equipment. These pins do not move. However, many pins must be retracted from the completed sheet metal assembly so that it can progress to the next station in the manufacturing process. Such locating pins are mounted on linear actuators which move them between extended and retracted positions. Locating pins which are retractable are subject to side to side deflection when they are in their extended positions. This deflection must be minimized if the sheet metal is to being reliably and repeatably located in the proper position. This goal is especially difficult to achieve during a welding process when heat from welding guns causes the sheet metal to expand.
Equipment used to support sheet metal during sheet metal welding operations often has limited space available for locating pins and their actuators. Therefore, it is often necessary for some of the locating pins to be attached to their actuators with an offset in order to allow the pin to fit around some other part of the equipment. The requirement of using a locating pin which is offset from the center line of its actuator requires the actuator to be firmly held against rotation.
The welding environment is a very challenging one for precision actuators. Weld spatter consists of very hard particles ranging in size from molecular to 1/8 of an inch in diameter, or more. This range of size coupled with very high temperatures makes the welding environment a harsh one for a precision actuator. If spatter is able to enter an actuator, the very hard particles cause rapid wear and premature failure of the actuator.
Locating pin actuators have consisted of a bearing system and piston and cylinder arrangement. The bearing system consists of a relatively long bearing housing in which a shaft is fitted. At one end of the shaft there is a provision to attach a locating pin, at the other end of the shaft provision is made to attach the piston and cylinder assembly.
There are three commonly used methods of providing a non-rotating feature to the shaft and bearing housing assembly. In one, arrangement to shaft is made square with a matching square bearing housing. In another arrangement the shaft is round with a flat ground onto one side. A flat bearing is then installed in the side of the bearing housing to engage this flat. In all implementations of locating pin actuators, a rod scraper is provided in an attempt to keep the weld spatter from entering the bearing system.
Prior art locating pin actuators have some deficiencies. You order to achieve the necessary limitation on side to side deflection, the bearing the assembly must be relatively long. This long bearing can be expensive to fabricate, and it may be difficult to utilize in tight places.
The actuators which use a square shaft are hard to fabricate. Consistently matching a square bearing to a square shaft in a high production environment is technically challenging. In addition, there must be some clearance between the shaft in the bearing so that the actuator can function smoothly. When torque is applied to the output shaft, as from an offset locating pin, the four corners of the shaft will contact the bearing surface. This minimal surface area of contact causes the bearing housing to wear rapidly. In addition, a square shaft can be hard to seal with a rod scraper. Therefore, it is common to use a shroud attached to the shaft which completely covers the exposed bearing surface. The shroud occupies additional space, making this arrangement even more difficult to fit into cramped quarters.
Actuators that use a flat surface ground into a round shaft to limit rotation are easier to manufacture than the square shaft arrangement because the grinding process is controllable, even in a high production environment. However, actuators which use a flat surface on a round shaft to limit rotation tend to wear rapidly because there is only a single point of contact between the flat and the bearing when a torque is applied to the shaft.